1. Field of the Invention
This invention concerns miniature monolithic multilayered capacitors, and more particularly, concerns methods for adjusting or trimming the capacitance values of monolithic miniature capacitors.
2. Description of the Prior Art
One technique widely used for trimming a miniature multilayer capacitor employs an abrasive carried by a stream of air which cuts through the top of the capacitor down through many electrodes to form a code shpaed cavity with circular top. As the abrasive stream cuts its way through the active internal electrodes, the capacitance of the capacitor is diminished until the desired capacitance value is reached, at which time the abrasive stream is stopped. This technique has several disadvantages and difficulties. Typically many of the capacitors employ electrode films or plates which are spaced one mil apart. Thus many active areas of adjacent electrodes are exposed by the abrasion process. All the exposed adjacent areas have opposite polarities which affords many opportunities to short circuit. The abraded surfaces are quite rough and have embedded in them many foreign particles which affect desired electrical parameters and operating characteristics of the capacitors, in adverse and unpredictable ways. Sometimes almost half of the capacitor body is cut away to achieve the desired capacitance value. When so much material is removed, cracking often occurs, and the structure is weakened. Sometimes, glass or epoxy is poured into the hole which is used to seal (insulate) the exposed electrode edges. Since the ceramic dielectric is replaced by another material having different dielectric, mechanical and thermal characteristics troublesome problems in operation of the capacitors result. Often the high Q dielectric, or the dielectric chosen for its specific qualities is replaced by a different dielectric or non-dielectric material. This compromised the overall electrical properties and chosen parameters. Further complications are caused by the different materials, particularly at the interfaces, which can be sheared apart or separated during subsequent soldering, lead attachment, or other heat treatment operation. The joints are very fragile and fail when subject to mechanical shocks in operation. The nonhomogenous parts are also extremely vulnerable to thermal shock, which defect may be latent, so that the capacitor passes inspection by the manufacturer and ultimate user, but fails in circuit use.
It has been proposed for trimming purposes to use a laser beam to burn a hole in a capacitor to remove portions of the dielectric body and parts of adjacent electrode layers. This method has also proven to be troublesome. It is not possible to adjust precisely the desired change in capacitance value, and the invasive penetration of the dielectric body exposes the interior to ambient conditions which are not conducive to stable operation. When the holes are filled with glass, epoxy or other filler, the same objections and disadvantages are experienced as are mentioned above in connection with capacitors having holes formed by abrasion.